Method of treating semiconductor devices to improve lifetime

ABSTRACT

1. IN A PROCESS FOR MANUFACTURING A SEMICONDUCTOR DEVICE, COMPRISING THE STEPS OF: PROVIDING A SUBSTRATE HAVING A NUMNER OF OPERATING SEMICONDUCTOR REGIONS FORMING AT LEAST ONE ACTIVE SEMICONDUCTOR ELEMENT, AT LEAST ONE OF SAID REGIONS BEING CONTIGUOUS WITH A GIVEN SURFACE OF SAID SUBSTRATE; FROMING A LAYER OF INSULATING MATERIAL ON SAID GIVEN SURFACE OVERLYING AT LEAST A PART OF SAID AT ONE REGION, SAID DEVICE INCLUDING AT LEAST ONE DELETERIOUS METAL INGREDIENT; EXPOSING SAID LAYER TO AN ATMOSPHERE COMPRISING A HYDROGEN HALIDE; AMD HEATING SAID SUBSTRATE TO A GIVEN TEMPERATURE SUFFICIENT TO CONVERT SAID METAL TO THE METAL HALIDE AND TO VOLATILIZE THE HALIDE AT THE EXPOSED SURFACE OF SAID INSULATING LAYER, THEREBY ESTABLISHING A GRADIENT FOR OUT-DIFFUSION OF SAID METAL FROM SAID DEVICE TOWARD SAID EXPOSED SURFACE, THE IMPROVEMENT WHEREIN SAID ATMOSPHERE IS MAINTAINED SUBSTANTIALLY FREE OF WATER VAPOR.

April 8, 1975 P. HEIMAN 21 A1. Re. 28,386

IIETHOD OI TREATING SEMICONDUCTOR DEVICES TO IHPRD E LIFETIME ori inalFiled April 11, 1968 Ward awn/r Z/Mr M146? IN VEN TORS Frederic .QfeimanQaul 9L obmson WM BY ATMUEY "United States Patent Re. 28,386 ReissuedApr. 8, 1975 28,386 METHOD OF TREATING SEMICONDUCTOR DEVICES TO IMPROVELIFETIME Frederic P. Heiman, East Brunswick, and Paul H. Robinson,Trenton, N.J., assignors to RCA Corporation Original No. 3,556,880,dated Jan. 19, 1971, Ser. No. 720,538, Apr. 11, 1968. Application forreissue Dec. 22, 1972, Ser. No. 317,743

Int. Cl. H011 7/34, 7/44 US. Cl. 148-191 14 Claims Matter enclosed inheavy brackets [II appears in the original patent but forms no part ofthis reissue specification; matter printed in italics indicates theadditions made by reissue.

ABSTRACT OF THE DISCLOSURE The stability and lifetime of a devicecomprising a semiconductor body covered by an insulating layer isimproved by thermally growing at least part of the insulating layer inan atmosphere comprising oxygen and hydrogen chloride, and substantiallyfree of water vapor. The device is heated in this atmosphere toestablish a gradient for out-dilfusion of certain deleterious metalsfrom the device.

BACKGROUND OF THE INVENTION This invention relates to a method fortreating semiconductor devices so as to improve the carrier lifetimethereof.

Many semiconductor devices include at least one region of semiconductormaterial covered by an overlying layer of insulating material. Inparticular, metal-oxide-semiconductor field effect devices, as well asplanar devices (in which a protective insulating layer overlies thesemiconductor surface at points where one or more P-N junction regionswithin the semiconductor material extend to the surface), employ thisconstruction.

In many semiconductor devices of this type, it is important thatinstabilities due to surface states at the semiconductor-insulatorinterface and to trapped charges in the insulating layer besubstantially eliminated. This requirement is especially severe in fieldeffect devices employing the insulating layer as a biased dielectric.

In other applications, such as the recently introduced silicon vidiconstructure (see. e.g., E. I. Gordon, A Solid-State Electron Tube for thePicturephone Set. Bell Laboratories Record. June 1967, pp. 1759), it isimportant that the semiconductor material exhibit a relatively longcarrier lifetime. A technique has been developed for improving thestability of semiconductor-insulator structures of the type described bytreating the insulating layer with an atmosphere which includes hydrogenchloride. This technique is described in US. patent application Ser. No.714,577, filed Mar. 21, 1968; now Pat. No. 3,556,879, by Alfred Mayer,and assigned to the assignee of the instant application.

The specific example set forth in application Ser. No. 714,577 involvessubjecting the semiconductor surface to an atmosphere comprising watervapor and hydrogen chloride. The water vapor rapidly oxidizes thesemiconductor surface to (i) form the (silicon dioxide) insulating layerby thermal oxidation of the underlying silicon material, so that theinsulating layer protects the semiconductor surface from undesirableetching by the hydrogen chloride gas, and the hydrogen chloride acts to(ii) convert certain deleterious metals to volatile chlorides at theexposed surface of the insulating layer, so that these chlorides leavethe exposed surface to establish a gradient for outdiffusion of suchdeleterious metals from the semiconductor device.

While the treatment process described in application Ser. No. 714,577substantially eliminates instability due to surface states and residualcharge or polarization, it produces only a limited improvement incarrier lifetime.

Accordingly, an object of the present invention is to provide animproved method for treating semiconductor devices of the type describedto considerably increase the carrier lifetime thereof.

SUMMARY OF THE INVENTION The invention is applicable to a semiconductordevice manufacturing process in which a layer of insulating material isformed on at least a part of an operating semiconductor region of anactive semiconductor element. The invention relates to an improvement inwhich the insulating layer is exposed to an atmosphere comprising ahydrogen halide, the atmosphere being maintained substantially free ofWater vapor.

The semiconductor device is heated in this atmosphere at a temperaturesufficient to convert a deleterious metal in the device to the metalhalide. The temperature is sufficient to volatilize the halide at theexposed surface of the insulating layer so as to establish a gradientfor out-diffusion of the deleterious metal from the semiconductor devicetoward the exposed insulating surface.

In the drawing:

FIG. 1 shows a silicon vidicon target structure manufactured accordingto the invention.

DETAILED DESCRIPTION In order to provide a substantial increase in thecarrier lifetime of a semiconductor material, it is necessary to removefrom the material any contaminants which degrade lifetime. Suchcontaminants are usually present in the form of heavy metals such asgold, copper and iron which act as trapping or recombination sites.

We have found that heat treatment of the semiconductor material in anatmosphere comprising hydrogen chloride and substantially free of watervapor produces a considerable improvement in lifetime.

In a particular process, a monocrystalline silicon wafer is cleaned byconventional methods. The wafer is then gas etched at 1100 C. in anatmosphere comprising hydrogen and including a volumetric concentrationof hydrogen chloride on the order of 1%. This etching treatment iscarried out for a sufiicient time to remove approximately 4 microns ofsilicon from the exposed surface of the wafer.

The wafer is then allowed to cool, and the hydrogen/ hydrogen chlorideatmosphere is replaced by dry oxygen. The silicon wafer is exposed tothe dry oxygen for approximately 3 minutes at a temperature on the orderof 1200 C. in order to form a thin protective thermally grown silicondioxide layer on the semiconductor surface. The purpose of this thininitial layer is to preclude undesirable etching of the silicon surfacewhen hydrogen chloride gas is subsequently introduced into the oxygenatmosphere.

After the initial 3 minute oxidation, a 1% volumetric concentration ofhydrogen chloride is introduced into the dry oxygen atmosphere, and thesemiconductor wafer is treated in this atmosphere at the 1200 C.temperature for approximately 4 hours. During this time, the thicknessof the oxide layer increases, and the hydrogen chloride acts to removedeleterious lifetime killing contaminants from thesemiconductor-insulator structure.

After the 4 hour treatment, the atmosphere is changed to helium in orderto remove any residual hydrogen chloride from the treated wafer.

The Wafer is allowed to cool and then annealed in a hydrogen atmosphereat a temperature on the order of 500 C. for a time on the order of 15minutes.

The annealing process improves performance of devices manufactured fromthe silicon/silicon dioxide composite by reducing or eliminating surfacestates at the semiconductor-insulator interface.

A capacitance-voltage curve obtained from a wafer processed according tothe method described above showed no measurable oxide charge, surfacestates or polarization.

The carrier lifetime of a wafer processed as described above wasmeasured by applying an evaporated aluminum electrode to the exposedsurface of the silicon dioxide layer, and subjecting the resultantstructure to a large voltage pulse, applied between the aluminumelectrode and the semiconductor material, in a polarity so as toestablish a large depletion region at the semiconductor surface adjacentthe electrode. The carrier lifetime was determined by measuring the timeconstant associated with relaxation of the depletion region to itsequilibrium condition. This measuring technique is described in detailin a paper by F. P. Heiman entitled On the Determination of MinorityCarrier Lifetime From the Transient Response of an MOS Capacitor,"published in the IEEE Transactions on Electron Devices, November 1967,p. 781.

This measurement technique indicated a carrier lifetime on the order ofto 300 microseconds, whereas the same measurement, when taken on a waferprocessed as described above but without the addition of hydrogenchloride to the oxygen atmosphere, yielded a lifetime of 0.2 to 1.0microsecond.

While the preferred embodiment of our process is directed to the use ofhydrogen chloride, any hydrogen halide which does not remove the silicondioxide insulating layer may be employed In particular, hydrogen bromideand hydrogen iodide may be substituted for the hydrogen chloride.

While we prefer to carry out the heat treatment step in the hydrogenchloride /oxygen atmosphere at a temperature in the range of 1000 to1200 0, this treatment may be satisfactorily accomplished attemperatures in the range of 800 to 1350" C. While our heat treatmentstep is carried out at an oxygen flow rate on the order of 1000 to 3000cc. per minute, and at atmospheric pressure, other pressures and flowrates may be employed. While the preferred volumetric concentration ofhydrogen chloride is on the order of 1%, good results are obtained witha volumetric concentration of 0.5%, and other concentrations in therange of 0.1 to 2% may be employed. On the other hand, a volumetricconcentration of 10% results in reaction of the hydrogen chloride withthe oxygen to produce water vapor; devices treated in the 10% hydrogenchloride atmosphere show negligible improvement in carrier lifetime, andpitting of the silicon surface.

The process of our invention may, e.g., be carried out in either aresistance heated or a cold wall (induction heated) furnace. Based uponthe aforementioned and other data which we have obtained, heat treatmentin an atmosphere comprising dry oxygen and hydrogen chloride, theatmosphere being substantially free of water vapor, results in anincrease of carrier lifetime by a factor of 10 to 1000 or more. Theabsence of water vapor during our heat treatment process was confirmedby monitoring the oxidation rate of the silicon semiconductor material,it being well known that silicon oxidizes much more rapidly in watervapor than in a dry oxygen atmosphere.

Our process is particularly applicable to the manufacture of a lightsensitive image pickup tube (hereinafter referred to as a siliconvidicon) which employs an electron beam addressed silicon diode array ofthe type shown in FIG. 1. Such a structure requires relatively high (onthe order of 10 micro seconds or more) carrier lifetimes in thesemiconductor material; such lifetimes may be reproducibly attained bythe process of our invention.

In a silicon vidicon tube, a target 1 is scanned by a low velocityelectron beam 2 emanating from a cathode 3. The electron beam 2 isformed, collimated, focussed, deflected and accelerated by a suitableelectrongun structure (not shown). Typically the electron beam 2 mayhave a circular cross-section with a diameter on the order of 1 mil.

The target 1 comprises a substrate 4 of monocystalline semiconductormaterial, preferably silicon, of one conductivity type into which alarge number of small regions 5 of opposite conductivity type arediffused. Preferably, the substrate 4 is of N type conductivity and thediffused regions 5 are of P type conductivity. The diffused P typeregions 5 are of a diameter substantially smaller than the diameter ofthe electron beam 2, so that the beam 2 subtends a number of the regions5, thus making it unnecessary to register the beam 2 with the individualregions.

Each of the diffused P type regions has a small P-N junction 6 to form adiode in conjunction with the substrate 4. The exposed surface of thesubstrate 4 adjacent the P type regions 5 is provided with a thinsilicon dioxide coating 7 which overlies and protects the P-N junctions6 where they extend to the semiconductor surface.

A thin surface layer 8 of relatively high electrical conductivity isdisposed adjacent the opposite surface of the substrate 4, i.e. thesurface which may be illuminated by a light image to be scanned. Theconductive layer 8 may comprise a layer of N+ conductivity type formedby diffusion of a suitable donor impurity into the substrate 4. Theconductive layer 8 and the substrate 4 are sufficiently thin so thatcarriers generated by the light incident upon the exposed surface of thelayer 8 may penetrate the substrate 4 to reach the P-N junctions 6.

The substrate 4 is supported by a ring 9 of relatively thicksemiconductor material, which may be secured to the inside envelope ofthe silicon vidicon tube.

Each of the P-N junctions 6 is reverse biased by means of (i) a voltagesource 10, which may typically have a value on the order of 10 volts,and (ii) a load resistor 11, which may typically have a value on theorder of several hundred thousand ohms. When the electron beam 2 is notscanning a particular P type region 5, the P-N junction 6 associatedwith that region is discharged by incident photons from the light image,the amount of discharge being dependent upon the photon flux. When thescanning electron beam 2 returns to this particular P-N junction,electrons flow to the P type region to provide a current which rechargesthe associated diode. This recharging current is directly related to thephoton flux (light intensity) from the light image incident upon the P-Njunction 6, and a corresponding voltage signal is developed across theload resistor 11. This signal is coupled to suitable amplified circuitryby means of a capacitor 12.

The incident light discharges the individual diodes by generatingelectron-hole pairs in the vicinity of the associated P-N junctions.These generated electrons and holes diffuse into the P-N junction regionand are swept across the junction by the associated space charge fieldtherein, thus serving to discharge the associated diodes. A number ofthe carriers created by the incident photons recombine and are lost, sothat they do not contribute to discharge of the associated diodes. Thisrecombination reduces the collection efiiciency of the target 1, anddirectly degrades the sensitivity of the Cilicon Vidicon.

The collection efficiency may be improved by increasing the bulk carrierlifetime and the surface recombination velocity of the semiconductormaterial comprising the target 1. Specifically, long carrier lifetimesand low recombination velocities provide high collection efiiciency andtherefore improve optical sensitivity.

The target 1 may be manufactured by providing a silicon substrate 4 of Ntype conductivity, having an N+ surface layer 8 diffused therein. Toform the P type regions 5, the corresponding surface of the substrate 4is coated with a thermally grown silicon dioxide layer 7, which maytypically have a thickness on the order of 0.5 to 1 micron. The silicondioxide layer 7 is grown in an atmosphere comprising dry oxygen and avolumetric concentration of hydrogen chloride on the order of 1%, in themanner previously described.

After the silicon dioxide layer has been grown, holes are etched thereinexposing small regions of the substrate 4. A thin glassy layercontaining a suitable acceptor impurity material is deposited on thesemiconductor surface, and subsequently heated to form the diffused Ptype regions 5. Preferably, borosilicate glass may be employed as theimpurity source. During or after the diffusion process, the borosilicateglass may be exposed to an atmosphere comprising hydrogen chloride, theatmosphere being substantially free of water vapor, in order to furtherimprove the carrier lifetime of the semiconductor material in the mannerpreviously described.

Thereafter, the portion of the borosilicate glass layer overlying theactive P type regions 5 may be removed by 'photoetching.

We claim:

1. In a process for manufacturing a semiconductor device, comprising thesteps of:

providing a substrate having a number of operating semiconductor regionsforming at least one active semiconductor element, at least one of saidregions being contiguous with a given surface of said substrate;

forming a layer of insulating material on said given surface overlyingat least a part of said at least one region, said device including atleast one deleterious metal ingredient;

exposing said layer to an atmosphere comprising a hydrogen halide; and

heating said substrate to a given temperature sufficient to convert saidmetal to the metal halide and to volatilize the halide at the exposedsurface of said insulating layer, thereby establishing a gradient forout-diffusion of said metal from said device toward said exposedsurface,

the improvement wherein said atmosphere is maintained substantially freeof water vapor.

2. The improvement according to claim 1, wherein said atmospherecomprises (i) substantially dry oxygen and (ii) hydrogen chloride,hydrogen bromide or hydrogen iodide.

3. The improvement according to claim 1, wherein said semiconductormaterial comprises silicon and said insulating layer comprises silicondioxide, at least a part of said silicon dioxide layer being thermallygrown during at least a part of said exposing step.

4. The improvement according to claim 3, wherein said atmosphereincludes dry oxygen and the volumetric concentration of said halide isless than 10%.

5. The improvement according to claim 4, wherein said halide compriseshydrogen chloride at a. volumetric concentration in the range of 0.1 to2%.

6. The improvement according to claim 4, wherein said halide compriseshydrogen chloride at a volumetric concentration on the order of 1%, saidatmosphere being maintained at normal atmospheric pressure.

7. The improvement according to claim 3, wherein said atmospherecomprises dry oxygen and hydrogen chloride, and said thermally grownsilicon dioxide is grown at a specified temperature in the range of 800'to 1350' C., said given temperature also being in the range of 800 to1350 C.

8. The improvement according to claim 7, wherein said temperatures arein the range of 1000 to 1200' C.

9. The improvement according to claim 7, comprising the additional stepof, after said insulating layer forming and exposing steps, annealingsaid device by heating said substrate in an atmosphere comprisinghydrogen gas.

10. The improvement according to claim 9, wherein said atmospherecomprises hydrogen and said annealing step is carried out at atemperature on the order of 500 C. for a time on the order of at least15 minutes.

11. A process for manufacturing an electron beam addressed semiconductordiode array target structure, comprising the steps of:

providing a substrate of monocrystalline semiconductor material of oneconductivity type, said substrate including at least one deleteriousmetal ingredient;

forming an insulating layer on one surface of said substrate, saidinsulating layer having a plurality of apertures therein exposingcorresponding areas of the substrate;

diffusing into said areas through said apertures a conductivity typedetermining impurity material to form in said areas a correspondingplurality of semiconductor regions of opposite conductivity type, with aP-N junction between each of said regions and said substrate;

exposing said insulating layer to an atmosphere comprising a hydrogenhalide and substantially free of water vapor; and

heating said substrate to a given temperature suflicient to convert saidmetal to the metal halide and to volatilize the halide at the exposedsurface of said insulating layer, thereby establishing a gradient forout-diffusion for said metal from said exposed surface.

12. A target manufacturing process according to claim 11, wherein saidsemiconductor material comprises silicon.

13. A target manufacturing process according to claim 12, wherein saidinsulating layer comprises borosilicate glass or silicon dioxide.

14. In a method of thermally growing a silicon dioxide layer on thesurface of a silicon material comprising the step of:

heating the silicon material to a temperature of between about 800' C.and I 350' C. in the presence of oxygen;

the improvement comprising the step of:

introducing into the oxygen gaseous hydrogen chloride in a concentrationbetween about 0.1% to 2.07 by volume relative to the oxygen to newtralize the electrical efiects of mobile ions in the silicon dioxidelayer.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 2,953,486 9/1960 Atalla 148-191 3,007,820 11/1961McNamara et al. 148-191 3,085,033 4/1963 Handelman 148-191 3,162,55712/1964 Brook et a1. 148-191 3,183,128 5/1965 Leistiko Ir., et a1.148-108 UX 3,243,323 3/1966 Corrigan et al. 148-188 UX 3,373,051 3/1968Chu et al. 11-48-15 UX 3,518,134 6/1970 Preist 148-15 X 3,562,033 2/1971Jansen et al. 148-189 CHARLES E. VAN HORN, Primary Examiner B. J.LEWRIS, Assistant Examiner U.S. Cl. X.R.

1. IN A PROCESS FOR MANUFACTURING A SEMICONDUCTOR DEVICE, COMPRISING THESTEPS OF: PROVIDING A SUBSTRATE HAVING A NUMNER OF OPERATINGSEMICONDUCTOR REGIONS FORMING AT LEAST ONE ACTIVE SEMICONDUCTOR ELEMENT,AT LEAST ONE OF SAID REGIONS BEING CONTIGUOUS WITH A GIVEN SURFACE OFSAID SUBSTRATE; FROMING A LAYER OF INSULATING MATERIAL ON SAID GIVENSURFACE OVERLYING AT LEAST A PART OF SAID AT ONE REGION, SAID DEVICEINCLUDING AT LEAST ONE DELETERIOUS METAL INGREDIENT; EXPOSING SAID LAYERTO AN ATMOSPHERE COMPRISING A HYDROGEN HALIDE; AMD HEATING SAIDSUBSTRATE TO A GIVEN TEMPERATURE SUFFICIENT TO CONVERT SAID METAL TO THEMETAL HALIDE AND TO VOLATILIZE THE HALIDE AT THE EXPOSED SURFACE OF SAIDINSULATING LAYER, THEREBY ESTABLISHING A GRADIENT FOR OUT-DIFFUSION OFSAID METAL FROM SAID DEVICE TOWARD SAID EXPOSED SURFACE, THE IMPROVEMENTWHEREIN SAID ATMOSPHERE IS MAINTAINED SUBSTANTIALLY FREE OF WATER VAPOR.